A suspension for a traveling vehicle does not provide a satisfactory driving feeling if it merely damps and absorbs vibrations produced on the body of the vehicle. In order to achieve further improvement in driving feeling, it is desirable to employ a suspension of active control which detects a behavior of the vehicle body to control the vehicle body so as to positively keep an appropriate posture thereof.
However, the active suspension system requires a hydraulic pump and a hydraulic valve and is complicated in construction of a controller itself for controlling them. Consequently, it requires a high cost.
Thus, increasing attention is paid to a so-called semi-active suspension system wherein a damper which has a variable damping force is employed so that the damping force characteristic is automatically controlled in response to an amplitude or a frequency of vibrations of the vehicle body.
FIG. 23 shows a basic construction of a conventional semi-active suspension system for a vehicle having four wheels. Referring to FIG. 23, the semi-active suspension system shown is constructed for a single wheel for the convenience of illustration and description, and actually, such semi-active suspension system is provided for each of the four wheels of the vehicle and only one controller is provided commonly for the semi-active suspension systems for the four wheels.
A suspension spring F, a known variable damping force damper G having a damping force which can be changed over between two hard and soft stages, and a vehicle height sensor H are interposed between a vehicle body D and a wheel E. An acceleration sensor J for detecting a vertical acceleration of the vehicle body D is provided on the vehicle body D.
A tire L serving as a spring element is present between the wheel E and the ground K.
A controller M includes a differentiator N, an integrator P and a calculation processing circuit Q. A signal from the vehicle height sensor H is inputted to the differentiator N while a signal from the acceleration sensor J is inputted to the integrator P. The calculation processing circuit Q processes output signals of the differentiator N and the integrator P and provides a damping force changing over signal to the variable damping force damper G so that the damping force may be changed over between two hard and soft stages on each of the extension and contraction sides of the variable damping force damper G as seen from FIG. 24.
As can be seen also from the basic construction described above, the controlling method of the conventional semi-active suspension system proceeds in the following manner.
In particular, where the displacements of the vehicle body D and the wheel E are represented by x and y, respectively, when the directions indicated by arrow marks in FIG. 23 are assumed to be in the positive in sign for the convenience of description, the relative velocity x-y between the vehicle body D and the wheel E is obtained by differentiating a signal x-y of the vehicle height sensor H by means of the differentiator N of the controller M. Meanwhile, the velocity x of the vehicle body D is obtained by integrating a signal x of the acceleration sensor J by means of the integrator P of the controller M.
Here, if attention is paid to the damping force of the variable damping force damper G acting upon the vehicle body D, when x&gt;0 wherein the vehicle body D is moving upwardly, if x-y&gt;0, then the variable damping force damper G is performing an extending operation, and accordingly, the damping force thus generated acts in the direction opposite to the direction of the movement of the vehicle body D. Consequently, the extension side damping force then acts as a vibration controlling force upon the vehicle body D. However, if x-y&lt;0, then the damping force generated acts in the same direction as the direction of the movement of the vehicle body D, and consequently, the compression side damping force then acts on the contrary as a vibration promoting force upon the vehicle body D.
Similarly, also when x&lt;0 wherein the vehicle body D is moving downwardly, in the case of x-y&gt;0 wherein the variable damping force damper G performs a compressing operation, the compression side damping force then acts as a vibration controlling force upon the vehicle body D, but on the contrary, in the case of x-y&lt;0 wherein the variable damping force damper G performs an extending operation, the extension side damping force then acts as a vibration promoting force upon the vehicle body D.
If this is represented in four quadrants taking x and x-y as the axis of ordinate and the axis of abscissa, respectively, then as seen in FIG. 25, the variable damping force damper G provides a vibration controlling force to the vehicle body D in the first and third quadrants, but provides a vibration promoting force to the vehicle body D in the second and fourth quadrants.
Then, the vibration controlling force acts so as to control vibrations of the vehicle body D upon vibration of the wheel E, and on the contrary, the vibration promoting force acts so as promote vibrations of the vehicle body D upon vibration of the wheel E.
Accordingly, as seen from FIG. 26, if the variable damping force damper G is changed over, in the first and third quadrants, to a hard mode in which the generated damping force is high or, in the second and fourth quadrants, to a soft mode in which the generated damping force is low, then vibrations of the vehicle body D can be controlled to a low level against vibrations of the wheel E.
If this is represented in equation, then EQU when x(x-y)&gt;0.fwdarw.hard damping force EQU when x-(xy)&gt;0.fwdarw.soft damping force
The driving feeling of the vehicle can be improved by controlling the variable damping force damper G in this manner.
However, with the conventional suspension system described above, while generally the sign of the positive or the negative of the vehicle x of the vehicle body D varies in a natural frequency, that is, in a number of variation per one hertz, which depends upon the mass of the vehicle body and the spring constant of the suspension spring F, the sign of the positive or the negative of the relative velocity x-y between the vehicle body D and the wheel E is varied at a considerably higher frequency by an instantaneous input of force from the ground K, resonance between the wheel E and the tire L, and so forth.
As a result, since the variable damping force damper G must necessarily be changed over at a considerably high frequency between the hard and soft levels, considerable durability is required for the variable damping force damper itself and also for a changing over actuator for it. Besides, if the actuator is not changed over at a high speed, then the controlling effect is deteriorated.
Further, since the relative velocity x-y between the wheel body D and the vehicle E must necessarily be detected upon controlling, the vehicle height sensor H must be provided for each of the wheels E. Accordingly, there is a problem that the entire system requires a high cost.